Many consumer devices are now constructed to generate and/or utilize digital data in increasingly large quantities. Portable digital cameras for still and/or moving pictures, for example, generate large amounts of digital data representing images. Each digital image may require up to several megabytes (MB) of data storage, and such storage must be available in the camera. To provide for this type of data storage application, the storage memory should be relatively low in cost for sufficient capacities of around 100 MB to one gigabyte (GB). The storage memory should also be low in power consumption (e.g. <<one Watt) and have relatively rugged physical characteristics to cope with the portable battery powered operating environment. For archival storage, data need only be written to the memory once. Preferably the memory should have a short access time (in the order of milliseconds) and moderate transfer rate (e.g. 20 Mb/s). Preferably, also, the storage memory should be able to be packaged in an industry standard interface module, such as PCMCIA, Smart Media, SD, Memory Stick or Compact Flash card.
One form of write-once compact information storage is shown in U.S. Pat. No. 6,055,180, granted to Gudesen et al. on Apr. 25, 2000 in which matrices of individually addressable cells are provided in layers between orthogonally arranged conductors. The cells may be comprised of cross-point diodes, OLEDs, bistable liquid crystal elements or other devices that change state with the introduction of heat and/or light.
Another application in portable devices for providing high density archival storage is described in co-pending U.S. patent application Ser. No. 09/875,356, entitled “Write-Once Memory”, the disclosure of which is hereby incorporated herein by reference. The memory system disclosed therein, referred to as portable inexpensive rugged memory (PIRM), aims to provide high capacity write-once memory at low cost for archival storage. This is realized in part by avoiding silicon substrates, minimizing process complexity and lowering areal density. The memory system includes a memory module formed of a laminated stack of integrated circuit layers constructed on plastic substrates. Each layer contains cross-point diode memory array, and sensing of the data stored in the array is carried out from a separate integrated circuit remotely from the memory module.
Because PIRM memory is relatively inexpensive, users will likely acquire a large number of PIRM modules with a variety of stored content. It is important to be able to fabricate and assemble memory modules in straightforward and relatively inexpensive processes that minimize the need for precision while maximizing information storage density and simplifying addressing, reading and writing functions.
In layered memory modules, such as described above, it is essential that the layers alternately containing a memory medium and multiple orthogonal conductors be precisely fabricated and aligned so that the cross points of conductors on either side of a memory layer are correctly aligned to form precise junctions for accurate diode production. If each layer is separately fabricated, an alignment structure is needed to assure that the layers are precisely stacked. There is a need for a simpler process for fabricating and assembling such layers to enhance precision in the stacking process while providing for a simplified fabrication process.
Moreover, to reduce manufacturing costs, diode memory elements are formed on plastic webs using roll to roll manufacturing rather than on silicon wafers. In order to meet the density requirements, photolithography techniques cannot be used. Instead the conductor wires may be formed by a method such as an emboss and lift-off process, as described in the co-pending U.S. patent application entitled “Write-Once Memory” referenced above. Accordingly the top wire grid must be made separately, instead of being fabricated on top of the bottom wire grid. This requirement mandates a method of manufacture in which the top and bottom wire grids are made separately but easily assembled without complicated alignment procedures.